1. Field of the Invention
The present invention relates to a golf ball excellent in flight performance and in durability, and more specifically, relates to an improvement in a resin composition to be used for an intermediate layer of a multi-layered golf ball.
2. Description of the Related Art
Examples of golf balls, based on the structure thereof, include a thread-wound golf ball, a two-piece golf ball covering a rubber-based core with a resin composition, and a multi-layered golf ball covering a core with an outer layer in which one or more of intermediate layer is formed between the core and a cover layer.
As a base resin used in an outer layer for covering a core of a multi-layered golf ball, such as a cover layer or an intermediate layer, ionomer resin and polyurethane are used. A thread-wound core or a vulcanized rubber-molded core is covered with a resin composition containing these resins as a main component.
In the above structure, a cover layer made of ionomer resin is widely used due to its excellent resilience and processing characteristics. However, such a cover layer having high stiffness and high hardness has a problem that it causes poor shot feeling, insufficient spin performance and poor controllability.
On the other hand, polyurethane is used as a base resin for constituting a cover layer, because the polyurethane provides improved shot feeling and spin performance as compared with ionomer resin. However, it cannot be said that the cover layer made of polyurethane provides resilience and abrasion resistance enhanced to satisfactory levels. In addition, when polyurethane is used, the golf ball becomes soft. Such a golf ball spins much at a driver shot and decreases a launch angle, resulting in a short ball distance.
As a countermeasure, for example, it has been proposed in a multi-layered golf ball to enhance resilience by increasing a hardness of an intermediate layer (Japanese Patent Publication No. 2910516).
However, an intermediate layer having too high hardness has a problem that a difference in hardness between a core and the intermediate layer is increased and thus durability is lowered. On the other hand, if controllability at an iron shot is focused, an increased spin rate is desired. However, if a hardness of an intermediate layer is raised in order to improve a ball distance as described above, it causes a contradictory problem to decrease the spin rate at an iron shot and thus to deteriorate the controllability.
In an attempt to solve the problems described above, it has been proposed to include various additives in an outer layer. For example, it has been proposed to include fibrous aluminum borate whiskers or organic short fiber in a cover layer (Japanese Laid-Open Patent Publication No. 10-137365 and Japanese Laid-Open Patent Publication No. 2002-136618). However, these fibrous substances have small specific surface areas. Because of such properties, although the fibrous substances are easily dispersed, the large-sized reinforcing materials come to scatter over the resin. Thus, when dispersed in the resin, the reinforcing materials have a small reinforcing effect. Therefore, in order to satisfy a desired mechanical property, it is required to blend a large amount of such reinforcing materials. As a result, since the resin content in the resin composition is reduced, it causes problems that high resilience being important as a golf ball is not obtained and abrasion resistance is lowered due to insufficient dispersion of the reinforcing materials.
Further, as an additive into an intermediate layer, it has been proposed to add inorganic fillers such as barium sulfate and titanium white (Japanese Laid-Open Patent Publication No. 2000-51396). However, since these fillers are not easily dispersed in the resin and the inorganic fillers raise the hardness, it is likely that a peeling between a core and an intermediate layer is caused and thus durability is lowered. In addition, the raised hardness lowers the controllability at an iron shot.
As a reinforcing material other than those described above, it has also been proposed to add nanocomposite material such as hydrotalcite or octosilicate as a filler into a resin composition of a cover layer or an intermediate layer (Japanese National Publication No. 2004-504900). Since the fillers of these types are formed into nano-sized fine particles, they are hard to be dispersed into resins. In particular, such fillers are difficult to be dispersed as particles into hydrophobic resins such as ionomer resin and polyurethane to be used for a cover layer or an intermediate layer. As a result, agglomerated particles are unevenly located in the resin, as is the case of the reinforcing materials having large particle diameters. Therefore, it has become apparent that, even if these reinforcing materials are used, neither the hardness of a desired level nor the sufficient durability is achieved.